diff --git a/llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h b/llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h
index 13082e440d4091..a3720eb3566838 100644
--- a/llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h
+++ b/llvm/include/llvm/CodeGen/GlobalISel/CSEInfo.h
@@ -120,6 +120,8 @@ class GISelCSEInfo : public GISelChangeObserver {
 
   void setMF(MachineFunction &MF);
 
+  Error verify();
+
   /// Records a newly created inst in a list and lazily insert it to the CSEMap.
   /// Sometimes, this method might be called with a partially constructed
   /// MachineInstr,
diff --git a/llvm/include/llvm/CodeGen/GlobalISel/GISelChangeObserver.h b/llvm/include/llvm/CodeGen/GlobalISel/GISelChangeObserver.h
index e5691cb35174a3..d8fe4b3103db9a 100644
--- a/llvm/include/llvm/CodeGen/GlobalISel/GISelChangeObserver.h
+++ b/llvm/include/llvm/CodeGen/GlobalISel/GISelChangeObserver.h
@@ -101,7 +101,7 @@ class GISelObserverWrapper : public MachineFunction::Delegate,
   void MF_HandleRemoval(MachineInstr &MI) override { erasingInstr(MI); }
 };
 
-/// A simple RAII based CSEInfo installer.
+/// A simple RAII based Delegate installer.
 /// Use this in a scope to install a delegate to the MachineFunction and reset
 /// it at the end of the scope.
 class RAIIDelegateInstaller {
@@ -113,5 +113,27 @@ class RAIIDelegateInstaller {
   ~RAIIDelegateInstaller();
 };
 
+/// A simple RAII based Observer installer.
+/// Use this in a scope to install the Observer to the MachineFunction and reset
+/// it at the end of the scope.
+class RAIIMFObserverInstaller {
+  MachineFunction &MF;
+
+public:
+  RAIIMFObserverInstaller(MachineFunction &MF, GISelChangeObserver &Observer);
+  ~RAIIMFObserverInstaller();
+};
+
+/// Class to install both of the above.
+class RAIIMFObsDelInstaller {
+  RAIIDelegateInstaller DelI;
+  RAIIMFObserverInstaller ObsI;
+
+public:
+  RAIIMFObsDelInstaller(MachineFunction &MF, GISelObserverWrapper &Wrapper)
+      : DelI(MF, &Wrapper), ObsI(MF, Wrapper) {}
+  ~RAIIMFObsDelInstaller() = default;
+};
+
 } // namespace llvm
 #endif
diff --git a/llvm/include/llvm/CodeGen/MachineFunction.h b/llvm/include/llvm/CodeGen/MachineFunction.h
index 1ec1b4b2864f9c..171abf05d67e86 100644
--- a/llvm/include/llvm/CodeGen/MachineFunction.h
+++ b/llvm/include/llvm/CodeGen/MachineFunction.h
@@ -72,6 +72,7 @@ class TargetRegisterClass;
 class TargetSubtargetInfo;
 struct WasmEHFuncInfo;
 struct WinEHFuncInfo;
+class GISelChangeObserver;
 
 template <> struct ilist_alloc_traits<MachineBasicBlock> {
   void deleteNode(MachineBasicBlock *MBB);
@@ -396,6 +397,7 @@ class MachineFunction {
 
 private:
   Delegate *TheDelegate = nullptr;
+  GISelChangeObserver *Observer = nullptr;
 
   using CallSiteInfoMap = DenseMap<const MachineInstr *, CallSiteInfo>;
   /// Map a call instruction to call site arguments forwarding info.
@@ -444,6 +446,10 @@ class MachineFunction {
     TheDelegate = delegate;
   }
 
+  void setObserver(GISelChangeObserver *O) { Observer = O; }
+
+  GISelChangeObserver *getObserver() const { return Observer; }
+
   MachineModuleInfo &getMMI() const { return MMI; }
   MCContext &getContext() const { return Ctx; }
 
diff --git a/llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp b/llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp
index b12407a4a4f46d..0e4fa219d988c1 100644
--- a/llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp
@@ -261,6 +261,39 @@ void GISelCSEInfo::releaseMemory() {
 #endif
 }
 
+Error GISelCSEInfo::verify() {
+#ifndef NDEBUG
+  handleRecordedInsts();
+  // For each instruction in map from MI -> UMI,
+  // Profile(MI) and make sure UMI is found for that profile.
+  for (auto &It : InstrMapping) {
+    FoldingSetNodeID TmpID;
+    GISelInstProfileBuilder(TmpID, *MRI).addNodeID(It.first);
+    void *InsertPos;
+    UniqueMachineInstr *FoundNode =
+        CSEMap.FindNodeOrInsertPos(TmpID, InsertPos);
+    if (FoundNode != It.second)
+      return createStringError(std::errc::not_supported,
+                               "CSEMap mismatch, InstrMapping has MIs without "
+                               "corresponding Nodes in CSEMap");
+  }
+
+  // For every node in the CSEMap, make sure that the InstrMapping
+  // points to it.
+  for (auto It = CSEMap.begin(), End = CSEMap.end(); It != End; ++It) {
+    const UniqueMachineInstr &UMI = *It;
+    if (!InstrMapping.count(UMI.MI))
+      return createStringError(std::errc::not_supported,
+                               "Node in CSE without InstrMapping", UMI.MI);
+
+    if (InstrMapping[UMI.MI] != &UMI)
+      return createStringError(std::make_error_code(std::errc::not_supported),
+                               "Mismatch in CSE mapping");
+  }
+#endif
+  return Error::success();
+}
+
 void GISelCSEInfo::print() {
   LLVM_DEBUG(for (auto &It
                   : OpcodeHitTable) {
@@ -370,6 +403,7 @@ GISelCSEInfo &
 GISelCSEAnalysisWrapper::get(std::unique_ptr<CSEConfigBase> CSEOpt,
                              bool Recompute) {
   if (!AlreadyComputed || Recompute) {
+    Info.releaseMemory();
     Info.setCSEConfig(std::move(CSEOpt));
     Info.analyze(*MF);
     AlreadyComputed = true;
diff --git a/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp b/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
index 62b903c30b89db..bdaa6378e901d8 100644
--- a/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelChangeObserver.cpp
@@ -38,3 +38,11 @@ RAIIDelegateInstaller::RAIIDelegateInstaller(MachineFunction &MF,
 }
 
 RAIIDelegateInstaller::~RAIIDelegateInstaller() { MF.resetDelegate(Delegate); }
+
+RAIIMFObserverInstaller::RAIIMFObserverInstaller(MachineFunction &MF,
+                                                 GISelChangeObserver &Observer)
+    : MF(MF) {
+  MF.setObserver(&Observer);
+}
+
+RAIIMFObserverInstaller::~RAIIMFObserverInstaller() { MF.setObserver(nullptr); }
diff --git a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
index 41800200384930..22a1eae3f4806a 100644
--- a/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp
@@ -2381,6 +2381,7 @@ bool IRTranslator::runOnMachineFunction(MachineFunction &CurMF) {
     WrapperObserver.addObserver(&Verifier);
 #endif // ifndef NDEBUG
     RAIIDelegateInstaller DelInstall(*MF, &WrapperObserver);
+    RAIIMFObserverInstaller ObsInstall(*MF, WrapperObserver);
     for (const BasicBlock *BB : RPOT) {
       MachineBasicBlock &MBB = getMBB(*BB);
       // Set the insertion point of all the following translations to
diff --git a/llvm/lib/CodeGen/GlobalISel/Legalizer.cpp b/llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
index e789e4a333dccc..0c0edc8a7b0c92 100644
--- a/llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/Legalizer.cpp
@@ -28,6 +28,7 @@
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/Error.h"
 #include "llvm/Target/TargetMachine.h"
 
 #include <iterator>
@@ -180,7 +181,7 @@ Legalizer::legalizeMachineFunction(MachineFunction &MF, const LegalizerInfo &LI,
 
   // Now install the observer as the delegate to MF.
   // This will keep all the observers notified about new insertions/deletions.
-  RAIIDelegateInstaller DelInstall(MF, &WrapperObserver);
+  RAIIMFObsDelInstaller Installer(MF, WrapperObserver);
   LegalizerHelper Helper(MF, LI, WrapperObserver, MIRBuilder);
   LegalizationArtifactCombiner ArtCombiner(MIRBuilder, MRI, LI);
   auto RemoveDeadInstFromLists = [&WrapperObserver](MachineInstr *DeadMI) {
@@ -305,6 +306,7 @@ bool Legalizer::runOnMachineFunction(MachineFunction &MF) {
     // We want CSEInfo in addition to WorkListObserver to observe all changes.
     AuxObservers.push_back(CSEInfo);
   }
+  assert(!CSEInfo || !errorToBool(CSEInfo->verify()));
 
   const LegalizerInfo &LI = *MF.getSubtarget().getLegalizerInfo();
   MFResult Result = legalizeMachineFunction(MF, LI, AuxObservers, *MIRBuilder);
@@ -324,5 +326,11 @@ bool Legalizer::runOnMachineFunction(MachineFunction &MF) {
     reportGISelFailure(MF, TPC, MORE, R);
     return false;
   }
+  // If for some reason CSE was not enabled, make sure that we invalidate the
+  // CSEInfo object (as we currently declare that the analysis is preserved).
+  // The next time get on the wrapper is called, it will force it to recompute
+  // the analysis.
+  if (!EnableCSE)
+    Wrapper.setComputed(false);
   return Result.Changed;
 }
diff --git a/llvm/lib/CodeGen/GlobalISel/Utils.cpp b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
index 73bc2a69596533..32f8666928355b 100644
--- a/llvm/lib/CodeGen/GlobalISel/Utils.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
@@ -12,6 +12,7 @@
 #include "llvm/CodeGen/GlobalISel/Utils.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/Twine.h"
+#include "llvm/CodeGen/GlobalISel/GISelChangeObserver.h"
 #include "llvm/CodeGen/GlobalISel/RegisterBankInfo.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
@@ -62,6 +63,15 @@ Register llvm::constrainOperandRegClass(
               TII.get(TargetOpcode::COPY), Reg)
           .addReg(ConstrainedReg);
     }
+  } else {
+    if (GISelChangeObserver *Observer = MF.getObserver()) {
+      if (!RegMO.isDef()) {
+        MachineInstr *RegDef = MRI.getVRegDef(Reg);
+        Observer->changedInstr(*RegDef);
+      }
+      Observer->changingAllUsesOfReg(MRI, Reg);
+      Observer->finishedChangingAllUsesOfReg();
+    }
   }
   return ConstrainedReg;
 }